This invention relates to gas turbine engines and, more particularly a thrust bearing loading arrangement for insuring unidirectional thrust on a ball bearing supporting one end of the rotor of a gas turbine engine.
The compressor discharge pressure of a gas turbine engine varies over a wide range through the normal operating range of a gas turbine engine from low or idling speed to maximum speed of the engine. This variation in operating conditions causes a substantial variation in the axial force exerted on the rotor of a gas turbine engine. Thus, at high operating speeds there may be a substantial axial load on the rotor in an aft or forward direction, thereby causing a corresponding axial force in the aft or forward direction on the thrust bearing supporting the rotor. On the other hand, at low or idling speeds, this axial force on the rotor is substantially reduced and may shift to the opposite direction, resulting in a condition known as crossover occurring at the time when the force exerted on the rotor, and correspondingly on the thrust bearing, changes from an aft direction to a forward direction or vice versa. Such crossover is undesirable because at the point of crossover the unloaded ball bearing is free to travel radially within its races. This unloaded condition of the ball bearing of the thrust bearing and the resultant radial movement of the unloaded ball bearing permits radial movement and/or orbiting of the rotor. For most efficient operation of a gas turbine engine, the clearance between the tips of the blades of the rotor and the surrounding stationary shroud is kept to a minimum. As a result, radial movement or orbiting of the rotor can cause the blade tips to engage the surrounding structure, resulting in wear of these tips and the surrounding structure. Such wear increases the compressor blade tip clearance resulting in a reduced stall margin and performance for the engine and the aircraft in which it is employed.
The unloaded thrust bearing also permits axial movement of the rotor. Since the compressor blade tips are slanted, this axial movement accentuates the increased clearance resulting from the aforementioned wear of the blade tips and surrounding structure, thereby compounding the problem.
One approach to the solution of this problem is to increase the thrust loading in an aft or forward direction so that, even under very low speed and idling conditions, the axial force on the rotor is still exerted in a consistent direction, thus providing adequate thrust load at idle and low speeds and preventing crossover. However, this approach introduces another problem in that there may then be an excessive thrust load at high speeds and high power, thereby reducing bearing life.
The present invention provides an arrangement for insuring against crossover at low speeds but avoiding any excessive thrust load at high speed operation of the engine.
Accordingly, it is an object of this invention to provide a thrust bearing loading arrangement which insures against crossover of the axial load exerted on the thrust ball bearing of a gas turbine engine, while at the same time avoiding excessive thrust load under conditions where the engine is operating at high power or maximum power.